A wide variety of polymers are available for a number of applications. Many of these polymers are modified by the addition of one or more additives to facilitate processing of polymers or to improve their properties. Most commercial polymers cannot be used without one or more additives used to modify the properties of the polymer in some way. Certain additives are used to overcome or enhance a polymer's properties in a particular application. For example, some polymers need to have their slip property enhanced in order to facilitate the formation of thin films of the polymer or in connection with the use of a polymer in a particular application. As a result, a processor may add one or more slip agents to the polymer during the extrusion process.
These slip agents and other additives are not typically compatible with the polymers they are extruded with and a portion of the slip agent migrates to the surface of the polymer film. In this example, the migration of the slip agent is a beneficial result. However, because many of the additives used in polymer processing are not compatible with the polymers, migration can occur with other polymer-additive blends. In these instances, the migration or bloom of the additive on the surface of the polymer film can be a defect. Other additives, such as antioxidants, light stabilizers and fire retardants are expected to remain in the polymer throughout its service life, which requires solubility. Precipitation of such additives on the polymer surface (blooming) is undesirable. Even in applications where migration is desired, there may be initially too much migration and not enough migration over time because the incompatibility of the polymer and the additive. As a result, the polymer can deteriorate or lose its properties over time because of the incompatibility of the polymer and the additive. Long-chain polar compounds like erucamide are widely used for anti-static, anti-fog, mould release and slip enhancing properties. These depend on incompatibility of the additive with the polymer and its migration to the surface. Proper performance of such additives implies that they should have a limited solubility in the polymer, and migrate to the surface at a suitable rate.
As additives migrate or bloom to the polymer surface, the ability to print, seal or coat the surface may suffer. This effects coating adhesion, lamination peels strength, and peel and blister resistance. Serious blooming can also affect surface aesthetics. White chalking on dark molded parts would be a typical example. Migration during processing can also cause buildup of additives on extruder dies and molding tool surfaces with obvious problems for the producer. Plasticizers are another type of additive that can migrate to the surface of a polymer. Flexible PVC and cellulose acetate are examples of plastics containing significant quantities of plasticizer and are the ones most likely to show effects of plasticizer migration. Loss of plasticizer causes the material to stiffen but not generally so much as to cause brittleness—flexible materials should therefore be supported in their natural shape so that they do not become ‘set’ in a distorted condition. Uneven loss of plasticizer from cellulose acetate plastic is responsible for distortion and is quite common with objects made from this material. It is often also, however, a sign that the cellulose acetate is also chemically deteriorating; the plasticizer becomes less compatible with the degrading polymer and migrates to the surface.
The additives industry has always been plagued by the disadvantage that composite mixtures of polymers and solid additives pose. These disadvantages arise both in processing, and in the trade offs that non-isotropic dispersion of the solid material in the plastic material impose. Non uniform mixing means that mechanical properties vary too much throughout the resulting product for many uses. It also means that fillers are often rendered less effective per percentage additive loads. This is not uncommon, for example, in the field of flame retardant packages.
Compatibilizing issues arise not just with additives but also with other materials added to polymers and blends of polymers. For example, mechanical enhancers are commonly added to polymers in certain applications. These enhancers can include glass fiber or mineral fibers. Other materials can be fillers that reduce the amount of an expensive polymer used in an application. Mineral fillers are commonly used to lower the cost of the polymer materials in an application. Another application of an additive is a thermoplastic based composite such as polymer concrete. The use of additives and fillers in polymers and polymeric blends can be problematical. Many additives are not compatible with the polymers so it is difficult to disperse the additives throughout the polymer.
These solid phase materials fillers and additives normally introduce compounding issues resulting from anisotropic dispersion and non uniform distribution throughout the polymer phase. Traditionally these problems have been solved by modifying the plastic processing equipment to a higher degree of sophistication and subsequent cost. Chemical approaches are by their very nature, selective in the materials with which they are effective, and thus limited in scope.
Block copolymers and silanes have been used to attempt to better compatiblize the introduction of fillers into a thermoplastic polymer; but these approaches are most often preceded by chemical processing steps of the filler upstream from their addition into the thermoplastic polymer, and in this key respect do not constitute prior art since they can be classed as chemical compatibilization relying upon multi-step processing before being used in the final melt polymer processing.
U.S. Pat. No. 6,239,196 discloses a process for preparing blends of a filler and a polymer. In the process of the '196 patent a filler of solid particles is dispersed in a polymer by extrusion of a composition prepared by mixing an aqueous suspension of said filler with particles or granules of said polymer at a temperature below melting point of the polymer. The mixing is preferably carried out before entry into the region for melting the polymer in the extruder and the extrusion is carried out at a temperature sufficient to melt the polymer and insufficient to cause the filler of solid particles to melt. The water initially present in the composition is removed under the effect of the heat partly at the extruder inlet and the remainder at the degassing vent or vents situated along the extruder. The '196 patent relies upon the use of an aqueous suspension, and is limited in its range of materials. Although much has been published about the use of organically modified nanoclays in the nanocomposite field, little attention if any has been paid the transformation of interstitial surface energy between polymers and solids.
Common clays are naturally occurring minerals and are thus subject to natural variability in their constitution. The purity of the clay can affect final nanocomposite properties. Many clays are aluminosilicates, which have a sheet-like (layered) structure, and consist of silica SiO4 tetrahedra bonded to alumina AlO6 octahedra in a variety of ways. A 2:1 ratio of the tetrahedra to the octahedra results in smectite clays, the most common of which is montmorillonite. Other metals such as magnesium may replace the aluminium in the crystal structure. Depending on the precise chemical composition of the clay, the sheets bear a charge on the surface and edges, this charge being balanced by counter-ions, which reside in part in the inter-layer spacing of the clay. The thickness of the layers (platelets) is of the order of 1 nm and aspect ratios are high, typically 100-1500. The clay platelets are nanoparticulate. In the context of nanocomposites, it is important to note that the molecular weight of the platelets (ca. 1.3×108) is considerably greater than that of typical commercial polymers. In addition, platelets are not totally rigid, but have a degree of flexibility. The clays often have very high surface areas, up to hundreds of m2 per gram. The clays are also characterized by their ion (e.g. cation) exchange capacities, which can vary widely. One important consequence of the charged nature of the clays is that they are generally highly hydrophilic species and therefore naturally incompatible with a wide range of polymer types. A necessary prerequisite for successful formation of polymer-clay nanocomposites is therefore alteration of the clay polarity to make the clay ‘organophilic’. An organophilic clay can be produced from a normally hydrophilic clay by ion exchange with an organic cation such as an alkylammonium ion. Exfoliated clay differs from other flat particles such as mica or aluminum flakes in its thickness. The atoms within a single layer of clay are tightly bound together, but the forces between layers are relatively weak. It is therefore possible to insert molecules between the layers spreading them apart. Furthermore, under the correct physical and chemical conditions, one can completely separate one atomic layer from its neighbors above and below. Thus one has molecularly thin sheets that are typically about 10 Angstroms (or 1 nm) in thickness, and can be anywhere from 0.1 to 10 microns in extent. The use of such particles to reduce the permeability of polymers, as well as to improve mechanical properties is an important part of nanocomposite technology.
Accordingly, there is a need for a method of evenly dispersing a solid phase material throughout a polymer material.